1. Field of the Invention
This invention relates generally to golf balls with cores, for example, single-layer cores having a surface hardness equal to or less than the center hardness, that is a negative hardness gradient. The invention also includes golf balls containing dual-cores having an inner core and outer core layer. In one version, each of the inner and outer core layers has a negative hardness gradient. In another version, the inner core layer has a negative hardness gradient and the outer core layer has a positive hardness gradient. Preferably, the cores are made of a rubber composition comprising cycloalkene (polyalkenamer) rubber and more preferably polyoctenamer rubber.
2. Brief Review of the Related Art
Solid golf balls are typically made with a solid core encased by a cover, both of which can have multiple layers, such as a dual-core having a solid center and an outer core layer, or a multi-layer cover having an inner. Generally, golf ball cores and/or centers are constructed with a thermoset rubber, typically a polybutadiene-based composition. The cores are usually heated and cross-linked to create certain characteristics, such as higher or lower compression, which can impact the spin rate of the ball and/or provide better “feel.” These and other characteristics can be tailored to the needs of golfers of different abilities. From the perspective of a golf ball manufacturer, it is desirable to have cores exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell many different types of golf balls suited to differing levels of ability.
Heretofore, most single core golf ball cores have had a conventional hard-to-soft hardness gradient from the surface of the core to the center of the core, otherwise known as a “positive hardness gradient.” The patent literature contains a number of references that discuss a hard surface to soft center hardness gradient across a golf ball core.
U.S. Pat. No. 4,650,193 to Molitor et al. generally discloses a hardness gradient in the surface layers of a core by surface treating a slug of curable elastomer with a cure-altering agent and subsequently molding the slug into a core. This treatment allegedly creates a core with two zones of different compositions, the first part being the hard, resilient, central portion of the core, which was left untreated, and the second being the soft, deformable, outer layer of the core, which was treated by the cure-altering agent. The two “layers” or regions of the core are integral with one another and, as a result, achieve the effect of a gradient of soft surface to hard center.
U.S. Pat. No. 3,784,209 to Berman, et al. generally discloses a soft-to-hard hardness gradient. The '209 patent discloses a non-homogenous, molded golf ball with a core of “mixed” elastomers. A center sphere of uncured elastomeric material is surrounded by a compatible but different uncured elastomer. When both layers of elastomer are concurrently exposed to a curing agent, they become integral with one another, thereby forming a mixed core. The center of this core, having a higher concentration of the first elastomeric material, is harder than the outer layer. One drawback to this method of manufacture is the time-consuming process of creating first elastomer and then a second elastomer and then molding the two together.
Other patents discuss cores that receive a surface treatment to provide a soft “skin.” However, since the interior portions of these cores are untreated, they have the similar hard surface to soft center gradient as conventional cores. For example, U.S. Pat. No. 6,113,831 to Nesbitt et al. generally discloses a conventional core and a separate soft skin wrapped around the core. This soft skin is created by exposing the preform slug to steam during the molding process so that a maximum mold temperature exceeds a steam set point, and by controlling exothermic molding temperatures during molding. The skin comprises the radially-outermost 1/32 inch to ¼ inch of the spherical core. U.S. Pat. Nos. 5,976,443 and 5,733,206, both to Nesbitt et al., disclose the addition of water mist to the outside surface of the slug before molding in order to create a soft skin. The water allegedly softens the compression of the core by retarding crosslinking on the core surface, thereby creating an even softer soft skin around the hard central portion.
Additionally, a number of patents disclose multilayer golf ball cores, where each core layer has a different hardness thereby creating a hardness gradient from core layer to core layer. There remains a need, however, to achieve a single layer core that has a soft-to-hard gradient (a “negative” gradient), from the surface to the center, and to achieve a method of producing such a core that is inexpensive and efficient. A core exhibiting such characteristics would allow the golf ball designer to create products with unique combinations of compression, “feel,” and spin.
Today, multi-piece solid golf balls are popular for several reasons including new manufacturing methods, availability and cost of raw materials, and playing performance properties of such balls. For example, three-piece solid golf balls having an inner core and outer cover with an intermediate layer disposed there between are commonly used by both professional and recreational golfers. In conventional multi-piece golf balls, the inner core is made commonly of a rubber material such as styrene butadiene, polybutadiene, poly(cis-isoprene), poly(trans-isoprene), or highly neutralized acid copolymers. Often, the intermediate layer is made of an olefin-based ionomer resin that imparts some hardness to the ball. These ionomer acid copolymers contain inter-chain ionic bonding and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. In recent years, there has been interest in using thermoplastic and thermosetting polyurethanes, polyureas, and hybrid compositions for the outer cover. The golf ball industry is looking to develop multi-piece balls having high resiliency as well as a soft feel. Balls having a high resiliency tend to reach a high velocity when struck by a golf club. As a result, the ball tends to travel a greater distance which is particularly important for driver shots off the tee. Meanwhile, the soft feel of the ball provides the player with a more enjoyable sensation when he/she strikes the ball with the club. The player senses a more natural feeling and control over the ball as the club face makes impact.
In turn, manufacturers of golf balls are looking at a wide variety of compositions for making multi-piece golf balls. For example, Kim et al., U.S. Pat. No. 7,528,196 and U.S. Patent Application Publication US 2009/0191981 disclose a golf ball comprising a core, cover layer, and optionally one or more inner cover layers, wherein at least one portion of the ball comprises a blend of a polyalkenamer and polyamide. The polyalkenamer/polyamide composition contains about 2 to about 90 weight % of a polyalkenamer polymer and about 10 to about 98 weight % of a polyamide. The '196 patent and '981 Published application further disclose that the polyalkenamer/polyamide composition may be blended with other polymers including polybutadiene, polyisoprene, polychloroprene, polybutylene, and styrene-butadiene rubber prior to molding. However, neither the '196 patent nor '981 Published application discloses a dual-core having an inner core and surrounding outer core layer, wherein the inner core has a zero or negative hardness gradient, and the outer core layer has a zero; negative; or positive hardness gradient and the inner core and/or outer core is made of a polyalkenamer rubber composition.
In Voorheis et al., U.S. Pat. No. 6,767,940, a golf ball having a core, an intermediate layer, and a cover is disclosed. The core is formed from a composition containing an elastomeric polymer, free-radical initiator, and at least one stable free-radical. The stable free-radical increases the scorch time (time between start of reaction and onset of cross-linking) of the elastomeric polymer. The '940 patent discloses numerous materials that can be used to form the intermediate layer, which is distinguishable from the core, including natural rubbers; balata; gutta-percha; cis-polybutadienes; trans-polybutadienes; synthetic polyisoprenes; polyoctenamers; polypropylene resins; ionomer resins; polyamides; polyesters; urethanes; polyureas; chlorinated polyethylenes; polysulfide rubbers; and fluorocarbons.
In Sullivan et al., U.S. Pat. Nos. 6,783,468, 7,041,009, 7,044,864, 7,118,495, and 7,125,345, a golf ball having a low compression and high coefficient of restitution (COR) layer supported and reinforced by a low deformation layer is disclosed. The preferred polymeric composition for the high COR layer is a base rubber compound, a co-reaction agent, a halogenated organosulfur compound, and a co-crosslinking or initiator agent. The low deformation layer may be made of rigid plastics or polymers reinforced with high strength organic or inorganic fillers or fibers. In one embodiment, the golf ball comprises an innermost core, an outer core, and a cover. The inner core comprises a low deformation material and the outer core comprises a rubber composition. The patents disclose that natural rubbers, including cis-polyisoprene, trans-polyisoprene or balata, synthetic rubbers including 1,2-polybutadiene, cis-polybutadiene, trans-polybutadiene, polychloroprene, poly(norbornene), polyoctenamer and polypentenamer may be used for the outer core. However, there is no disclosure of forming a dual core, wherein the inner core has a positive hardness gradient and the outer core layer has a zero; negative; or positive hardness gradient, and the inner core and/or outer core is made of a polyalkenamer rubber composition.
In addition, Llort, U.S. Pat. No. 4,792,141 describes a balata-covered golf ball, where up to 40% of the balata used to form the cover has been replaced with polyoctenylene rubber. The golf ball contains a core and a cover wherein the cover is formed from a composition comprising about 97 to about 60 parts balata and about 3 to about 40 parts by weight polyoctenylene rubber based on 100 parts by weight polymer in the composition. The '141 patent discloses that using more than about 40 parts by weight of polyoctenylene produces deleterious effects.
One objective of the present invention is to develop compositions that can be used to make a core for a golf ball, wherein the core provides the ball with high resiliency along with a comfortable and soft “feel.” The present invention provides golf ball core compositions having such properties as well as other advantageous characteristics, features, and benefits.